1. Field of the Invention
The present invention relates in general to a rodless pump system for use in an oil well, for example, and in particular to a rodless pump system having a surface unit for applying fluid pressure to a fluid conduit and a subsurface pump responsive to the fluid pressure for lifting fluid from the well into the fluid conduit.
2. Description of the Prior Art
Presently, low pressure, non-flowing oil wells account for over about 90% of the oil wells in the United States. There are various means available for pumping these non-flowing oil wells, the most common of these pump means being the sucker rod type subsurface pump. Other types of pumps include electrical and hydraulic actuated subsurface pumps. A feature which is common to each of these subsurface pumps is that each utilizes a separate energy transmission mechanism other than the production fluid conduit for supplying the actuating energy to the pump, resulting in expensive energy transmission paths, requiring considerable maintenance and cost.
Although sucker rod type pumps are not the most energy efficient, they are probably the most reliable. However, sucker rod failures are still a major problem, as studies have shown that a sucker rod fails an average of once every two years. These failures result in significant repair and maintenance costs.
There have been several attempts to provide a rodless subsurface pump system which does not require an energy transmission mechanism for activating the pump. This type of pump system typically includes a surface unit which is connected to the subsurface pump by a single fluid conduit. The surface unit activates the subsurface pump by applying pressure to the fluid in the conduit to compress a spring means in the pump and displace a slidable piston to draw fluid from the well into a pump chamber. When the surface unit releases the fluid pressure, the spring means of the subsurface pump will displace the piston and lift the fluid in the pump chamber into the fluid conduit. Such systems are disclosed in U.S. Pat. Nos. 2,058,455; 2,123,139; 2,126,880 and 2,508,609.
However, these pressure activated subsurface pump systems have some inherent problems. When fluid pressure is applied to the fluid conduit, the actual energy applied to the system is much greater than the energy supplied to the subsurface pump. Since thousands of feet typically separate the surface unit and the subsurface pump, considerable work is done compressing the fluid in the conduit, ballooning the conduit, and moving fluid to compress the subsurface pump spring. In these systems, typically more energy is consumed in compression and ballooning than is used to energize the subsurface pump. Because fluid pressure is simply relieved typically to atmospheric pressure while taking production, the compression energy is wasted.